The present invention relates to carbonaceous material for use in energy storage devices, specifically to the use of doped carbon foam for use in such devices, and more specifically to phosphorus (P) doped carbon foams for use, for example, as a lithium intercalation anode in a rechargeable lithium ion battery, or as an electrode in a supercapacitor. Doping in the context of this patent refers to donor or acceptor dopants, as opposed to intercalate doping, as occurs with intercalation of lithium ions into carbon.
As the result of the recent trend toward miniaturization of electronic components and the recognized benefits of rechargeable energy storage devices, substantial effort has been directed toward the development of small rechargeable cells such as the non-aqueous electrolyte lithium cell. In recent times, rechargeable lithium cells of various types have been extensively studied for practical utilization. However, rechargeable, nonaqueous electrolyte cells with lithium metal negative electrodes have had the disadvantages of long charging times, poor quick charging properties, short cycle life and serious safety problems due to uneven electroplating of lithium metal during cycling.
More recently, several groups have developed rechargeable battery systems based on lithium intercalation to address many of the problems inherent in lithium metal batteries. Although the lithium ion system has a lower theoretically achievable energy density, it is inherently a safer and more rechargeable system due to the different energy storage mechanism. One solution is exemplified by U.S. Pat. No. 4,668,595 issued May 26, 1987 to A. Yoshino et al., which discloses the doping in a wide variety of carbons, formed from carbon powders, carbon blacks and carbonized polymeric fibers.
Another approach has been the incorporation of phosphorus in carbonized materials such as resins, certain hydrocarbons and some polymeric compounds from furfuryl alcohol precursors, as exemplified by U.S. Pat. No. 5,093,216 issued Mar. 3, 1992 to H. Azuma et al. In these instances there has been seen intercalation of lithium in amounts higher than that which can be the theoretical limit (LiC.sub.6), and this can lead to substantially higher energy densities then those previously achieved, when incorporated in a lithium ion battery.
The above described prior art rely primarily on the use of carbon electrodes which have been formed by pressing carbon powders with binder to form a solid carbon electrode. Related embodiments may rely on carbonization and compaction of fibers, cellulose, etc. These are distinct from the present invention, which utilizes monolithic, open-cell carbon foams as electrodes. Carbon foams have distinct advantages for use in energy storage systems, including easily controllable porosity and density, and high conductivity and strength. The morphology of the material may also be controlled by varying the pre-cursor fabrication conditions. This may include fabrication of foams in microspheres of varying morphology.
In recent years, various types of carbon foams have been fabricated, including low-density carbon foams such as exemplified by U.S. Pat. No. 4,873,218 issued Oct. 10, 1989 and U.S. Pat. No. 4,997,804 issued Mar. 5, 1991, each to R. W. Pekala. These carbon foams have a density of .ltoreq.100 mg/cc and cell size of .ltoreq.0.1 microns. This concept has been further extended, by the present inventors, to higher density foams (0.1 to 1.0 gm/cm) described and claimed in U.S. patent application Ser. No. 07/822,438 filed Jan. 17, 1992, now U.S. Pat. No. 5,260,855, issued Nov. 9, 1993. Also, more recently, the concept has been developed of loading carbon foams with different types of materials, depending on the application, but this loading of carbon foams did not modify the localized structure to change the characteristics of the foam, as would doping of these foams with a selected material. In doping, materials would be distributed and bonded into the carbon matrix on a molecular level.
While the doping of a carbonized material, as in above-referenced U.S. Pat. No. 5,093,216, has provided increased energy densities, doped carbon foams, as an anode, can provide higher energy and power densities, due to the improved electronic conductivity which results from their monolithic structure. This reduces, or eliminates the requirement for a metallic current collector, thereby reducing the weight of the packaged battery. Dopant species can be incorporated into the carbon as substitutional atoms (dopants), during the polymerization and pyrolysis of the material. The resulting carbon material, of unspecified form (foam, foam microspheres or other), differs in its electrochemical characteristics from a pure carbon sample. These differences can affect 1) the voltage at which lithium is electrochemically intercalated into the carbon, 2) the capacity of the carbon for intercalation lithium, 3) the charging or discharging rate of lithium intercalation, 4) the cycleability of the device, and 5) the reversibility of the intercalation process.
It has been recognized by the present inventors that doping carbon foams with selected materials ("dopants" P, B, As and Sb) will modify the localized graphitic structure to improve intercalation characteristics (i.e. d.sub.002 plane spacing), and thus provide increased intercalation of lithium ions and hence improve the potential energy density when used as an electrode in a rechargeable battery. Thus, this recognition by the inventors provides a method by which the energy storage capacity per unit weight in lithium cells, for example, could be increased by incorporating the dopants into the carbon foams and thereby produce the increased densities provided by above-referenced U.S. Pat. No. 5,093,216 while reducing the weight of the cell. Therefore, the present invention fills a need in the field of miniaturized nonaqueous electrolytic cells, while providing the needed high energy density.